1. Field of the Invention
The present invention relates to a process for inhibiting the corrosion of masses of metal in contact with acid baths containing ferric ions, for example an industrial plant subjected to a descaling operation, or metallurgical products subject to a pickling operation.
2. Description of the Background
It is known to treat metal plants with acid baths, especially for the purpose of pickling or descaling them. These pickling and descaling methods are widely employed in industry, where they are used particularly for descaling steel boilers and for removing encrustation from crystallisation reactors such as steel or cast iron columns employed for the crystallisation of sodium bicarbonate in the ammonia soda manufacturing process.
It is common practice moreover, in the conversion of metallurgical products, to subject the latter to. a final pickling operation in an acid bath. This is the case mainly with rolling products made of ordinary steel, and more particularly with those produced by cold rolling such as strips and drawn wires.
It is general practice to incorporate corrosion inhibitors in the baths. Corrosion inhibitors which are ordinarily employed for this purpose are alkylpyridinium chlorides, hexamethylenetetramine and organic compounds containing sulphonium groups. However, it has been observed in practice that, despite the presence of corrosion inhibitors in the acid baths, the latter nevertheless cause a rapid corrosion of the masses of metal which are treated. This corrosion may be attributed to the presence of oxidising agents in the bath, chiefly ferric ions. To overcome this disadvantage it has already been recommended to add reducing agents to the bath, for example stannous chloride, in addition to the corrosion inhibitors. The use of reducing agents in the bath is generally costly, particularly in the case of baths which are not deaerated, because a significant loss of reducing agents is then recorded, due to their premature consumption by the oxygen in the bath, this consumption being proportionately higher the higher the temperature. Furthermore, this is followed by an increase in bath contamination by reducing agents some of which, in particular stannous ions, are generally deemed to be ecologically dangerous, especially when the residual baths are discharged into lakes or water courses.